1. Field of the Invention
The present invention relates to an antenna device which is contained in radio equipment such as a portable telephone, and so forth, and to radio equipment provided with the same.
2. Related Art
FIG. 18 schematically shows an example of a dual band type antenna device. An antenna device 40 shown in FIG. 18 can transmit or receive radio waves in two different frequency bands, and comprises an antenna conductor portion 41, an inductor portion 42, a change-over circuit 43 for changing the inductance of the inductor portion 42, and an inductor 44 which functions as a matching circuit.
The antenna conductor portion 41 has, for example, a form of a conductor wire member such as a whip antenna or the like, a conductor film formed on the surface of a rectangular parallelepiped substrate, and so forth. The inductor portion 42 is connected in series with the power supply side of the antenna conductor unit 41, and the inductance component of the inductor portion 42 is coupled to the antenna conductor unit 41. The inductance of the antenna conductor portion 41 can be equivalently changed by changing the inductance of the inductor portion 42 by means of the change-over circuit 43. Thus, the inductor portion 42 can resonate in two different frequencies when the changing is carried out. Accordingly, the antenna device 40 can transmit and receive radio waves in the two different frequency bands.
However, for the above-described configuration of the antenna device 40, a complicated change-over circuit as shown in FIG. 18 is needed, when two frequency bands significantly distant from each other, such as a PDC (personal digital cellular) 800 MHz band and a PDC 1.5 GHz band, are changed. Thus, problems arise in that the number of parts of the change-over circuit 43 is large, increasing the cost, the conduction loss in the change-over circuit 43 is large, reducing the antenna sensitivity, and so forth.
Accordingly, it is an object of the present invention to solve the above-described problems and provide an antenna device which can transmit and receive radio waves in two different frequency bands and is inexpensive, and radio equipment including the same.
To solve the above-described problems and achieve the above object, according to the present invention, there is provided an antenna device which can transmit and receive radio waves in two different frequency bands, comprising an antenna conductor portion having a resonance frequency which is lower than the center frequency in the higher frequency band for carrying out the transmission and reception of the radio waves and is higher than the center frequency in the lower frequency band for carrying out the transmission and reception of the radio waves, and an LC parallel resonance circuit connected in series with the power supply side of the antenna conductor portion, the LC parallel resonance circuit being configured so as to resonate at a frequency nearly equal to the center frequency in the lower frequency band, causing the antenna conductor portion to resonate at the center frequency in the lower frequency band, and so as to provide a capacitance for causing the antenna conductor portion to resonate at the center frequency in the higher frequency band.
Preferably, the antenna conductor portion comprises a conductor sheet member or conductor wire member having an electrical length equal to about one quarter of the wavelength of a radio wave having a frequency between the center frequency in the higher frequency band and the center frequency in the lower frequency band.
Also, preferably, the antenna conductor portion comprises a conductor sheet member, and has an electrical length equal to about one quarter of the wavelength of a radio wave having a frequency between the center frequency in the higher frequency band and the center frequency in the lower frequency band.
Preferably, the antenna conductor portion comprises a combination of the conductor portion for transmitting and receiving a radio wave, formed on a substrate, and a conductor sheet member or conductor wire member electrically connected to each other, and the combination has an electrical length equal to about one quarter of the wavelength of a radio wave having a frequency between the center frequency in the higher frequency band and the center frequency in the lower frequency band.
Also, preferably, the capacitor portion constituting the LC parallel circuit is configured so as to contain at least a varicap diode having a parasitic capacitance variable depending on applied voltage, and a voltage input portion for determining the parasitic capacitance of the varicap diode is electrically connected to the capacitor portion.
More preferably, a change-over circuit for changing the inductance of the inductor portion constituting the LC parallel resonance circuit in plural steps to vary and set the lower frequency band is connected to the inductor portion constituting the LC parallel resonance circuit.
Preferably, the inductor portion comprises plural inductors connected in series to each other, a bypass conduction path is provided in parallel to at least one of the plural inductors constituting the inductor portion, and a switching portion for controlling the conduction on-off of the bypass conduction path whereby the conduction on-off of the inductor connected in parallel to the bypass conduction path is incorporated in the bypass conduction path, the bypass conduction path and the switching portion constitute the change-over circuit for changing the inductance of the inductor portion to vary and set the lower frequency band.
Radio equipment according to the present invention is characterized in that the equipment includes one of the above-described antenna devices.
According to the present invention, the LC parallel resonance circuit is connected in series with the power supply side of the antenna conductor portion. Since the LC parallel resonance circuit resonates at a frequency nearly equal to the center frequency in the lower frequency band for transmitting and receiving a radio wave, an inductor component, caused by the LC parallel resonance circuit, is rendered to the antenna conductor portion, and thereby, the antenna conductor portion resonates at the center frequency in the lower frequency band to carry out the operation as an antenna.
The antenna conductor portion has a resonance frequency which is lower than the center frequency in the upper frequency band. The LC parallel resonance circuit presents a capacitive impedance characteristic in the upper frequency band higher than the resonance frequency of the circuit. Thus, the capacitance of the LC parallel resonance circuit is connected in series with the power supply side of the antenna conductor portion in the frequency band higher than the resonance frequency of the LC parallel resonance circuit, so that the inductance of the antenna conductor portion is reduced. As a result, the antenna conductor portion resonates at a frequency higher than the resonance frequency of the antenna conductor portion itself. Accordingly, the antenna conductor portion can resonate at the center frequency in the higher frequency bands and thus, can operate as an antenna by setting the circuit constants of the LC parallel resonance circuit so that the antenna conductor portion can resonate at the center frequency in the higher frequency band.
The antenna conductor portion can transmit and receive radio waves in the two different frequency band, due to the simplified configuration in which the LC parallel resonance circuit is connected in series with the antenna conductor portion without need of a circuit for changing the upper and lower frequency bands.
In the arrangement of the present invention, no complicated circuits for changing the upper and lower frequency bands are provided as described above. Thus, the circuit configuration becomes simple, and the conduction loss can be reduced. Accordingly, the antenna sensitivity can be enhanced, and increase in cost can be prevented.